: The steroid hormone control of cell-cell interactions is a potentially important regulatory mechanism to facilitate coordinated and intricate changes in plasma membrane composition, function and intracellular signaling. It has been previously documented that glucocorticoids, one class of steroid hormones, induce tight junction formation and cell polarity in cultured monolayers of tumorigenic and nontumorigenic mammary epithelial cells, implicating this lactogenic steroid as a key in vivo regulator of cell-cell contact during differentiation of the mammary gland. Glucocorticoids stimulate the recruitment of the tight junction proteins, adherent junction proteins and two signaling molecules to the sites of cell-cell contact. This junctional reorganization process is followed by a distinct Ras-dependent step that results in formation of highly sealed tight junctions. Several critical regulatory components of the glucocorticoid signaling cascade that are required for the steroid control of tight junction dynamics in mammary epithelial cells, have been uncovered. An early event in the cascade is the glucocorticoid stimulated expression of the Id-1 transcriptional regulator. It is postulated that Id-1 alters the expression of a specific set of target genes that ultimately regulate mammary cell-cell interactions. A novel glucocorticoid-regulated ankyrin-related occludin tail-interacting protein (named GAO) has been cloned using a yeast two-hybrid screen. It is proposed that GAO links the steroid induced cascade to the tight junction structural proteins. One aim of this proposal is to characterize the structure/function properties of GAO and determine its role in the control of tight junction dynamics by expression of wild type, mutated and antisense GAO sequences in mammary epithelial cell tumor cells and by the in vitro manipulations of permeabilized cells. The other aim of this proposal is to identify and functionally characterize the cellular targets of Id-1 by a differential DNA microarray screen between matched pairs of mammary epithelial cells with selective alterations in Id-1 expression. Cell-cell interactions play a critical role in controlling the growth and differentiation of normal tissue, whereas, the malignant phenotype is associated with disruptions in this process. Information from the proposed studies may eventually prove to be clinically useful in the potential development of therapeutic strategies to control physiologic disorders and neoplasias with apparent alterations in plasma membrane function, adhesion and intracellular communication.